Ultraviolet

Ultraviolet radiation, often shortened to UV, is a type of electromagnetic radiation with wavelengths between 100 and 400 nanometers. It has shorter wavelengths than visible light but longer than X-rays. UV light is found in sunlight, making up about 10% of the Sun's total energy output. It can also be created by electric arcs, special lights like mercury-vapor lamps, and black lights.

UV radiation has both helpful and harmful effects. For humans, moderate exposure to UV light helps our bodies make vitamin D, which is important for strong bones. However, too much UV can cause suntan, sunburn, and even increase the risk of skin cancer. The Earth's atmosphere protects us by filtering out much of the Sun's most harmful UV rays.

Even though we can't see UV light, some animals can. Insects, birds, and certain mammals can detect wavelengths that are just shorter than what humans see, allowing them to notice UV light. This special ability helps them find food, recognize mates, and navigate their world in ways we cannot.

Visibility

Humans cannot see ultraviolet light because our eyes block it and we do not have the right tools to detect it. Special parts of our eyes can sense some ultraviolet light, but it doesn’t focus well, making it look blurry. Some animals, like certain birds, can actually see ultraviolet light because their eyes are different from ours.

History and discovery

Ultraviolet means "beyond violet" because it has a higher frequency and shorter wavelength than violet light, which is the highest frequency color we can see.

This type of light was discovered in February 1801 by a German scientist named Johann Wilhelm Ritter. He noticed that invisible rays just beyond violet light could darken special paper more quickly than violet light itself. Later, scientists learned that ultraviolet light can kill bacteria and affect DNA. In 1893, another scientist discovered ultraviolet light with even shorter wavelengths, which is absorbed by air.

Subtypes

The electromagnetic spectrum of ultraviolet radiation (UVR), defined as 10–400 nanometers, can be divided into several ranges. One way to do this, recommended by the ISO standard ISO 21348, looks at different wavelength groups.

Ultraviolet can be detected by special photodiodes and photocathodes, and there are sensitive UV photomultipliers available. Tools like spectrometers and radiometers help scientists measure UV radiation, and silicon detectors can be used across the whole UV spectrum.

Name		Photon energy (eV, aJ)	Notes/alternative names
Abbreviation	Wavelength (nm)	Photon energy (eV, aJ)	Notes/alternative names

Ultraviolet A		3.10–3.94 0.497–0.631	Long-wave UV, blacklight, not absorbed by the ozone layer: soft UV.
UVA	315–400	3.10–3.94 0.497–0.631
Ultraviolet B		3.94–4.43 0.631–0.710	Medium-wave UV, mostly absorbed by the ozone layer: intermediate UV; Dorno radiation.
UVB	280–315	3.94–4.43 0.631–0.710
Ultraviolet C		4.43–12.4 0.710–1.987	Short-wave UV, germicidal UV, ionizing radiation at shorter wavelengths, completely absorbed by the ozone layer and atmosphere: hard UV.
UVC	100–280	4.43–12.4 0.710–1.987

Near ultraviolet		3.10–4.13 0.497–0.662	Visible to birds, insects, and fish.
NUV	300–400	3.10–4.13 0.497–0.662	Visible to birds, insects, and fish.
Middle ultraviolet		4.13–6.20 0.662–0.993
MUV	200–300	4.13–6.20 0.662–0.993
Far ultraviolet		6.20–10.16 0.993–1.628	Ionizing radiation at shorter wavelengths.
FUV	122–200	6.20–10.16 0.993–1.628	Ionizing radiation at shorter wavelengths.
Hydrogen Lyman-alpha		10.16–10.25 1.628–1.642	Spectral line at 121.6 nm, 10.20 eV.
H Lyman‑α	121–122	10.16–10.25 1.628–1.642	Spectral line at 121.6 nm, 10.20 eV.
Extreme ultraviolet		10.25–124 1.642–19.867	Entirely ionizing radiation by some definitions; completely absorbed by the atmosphere.
EUV	10–121	10.25–124 1.642–19.867

Far-UVC		5.28–6.20 0.846–0.993	Germicidal, with single exposure considered safe, causing minimal damage to DNA; potential risks with repeated application..
	200–235	5.28–6.20 0.846–0.993
Vacuum ultraviolet		6.20–124 0.993–19.867	Strongly absorbed by atmospheric oxygen, though 150–200 nm wavelengths can propagate through nitrogen.
VUV	10–200	6.20–124 0.993–19.867

Solar ultraviolet

The Sun sends out a special kind of light called ultraviolet (UV) radiation. This light has shorter waves than the light we can see with our eyes. In space, sunlight includes about 10% UV light.

Earth's air helps protect us by blocking most of this UV light. When the Sun is directly overhead, the air stops about 77% of the UV. The little UV that reaches us on the ground is mostly a type called UVA, with a tiny bit of UVB. This UV light helps plants grow because it affects their hormones. The air also creates an important layer called the ozone layer, which blocks even more of the Sun's strong UV light.

Blockers, absorbers, and windows

Ultraviolet absorbers are special molecules used in materials like polymers and paints to protect them from damage caused by UV light. These absorbers can wear down over time, so it’s important to check their levels in old materials.

In sunscreen, ingredients like avobenzone, oxybenzone, and octyl methoxycinnamate help block harmful UV rays. Some sunscreens also use minerals such as titanium dioxide and zinc oxide for protection. Clothing can also protect against UV light, with a rating called the ultraviolet protection factor (UPF), similar to sunscreen’s SPF rating.

Special glass, like that used in stained-glass windows, can prevent UV light from changing colors. Regular window glass lets some UV light through, but special types like fused quartz can block even more of it. There are also special glasses, like Wood's glass, that let UV light pass for certain uses, such as secret communications.

Artificial sources

Main article: Blacklight

Black lights are special lamps that give off a type of invisible light called ultraviolet (UV) light. These lamps are used to make certain materials glow in colorful patterns. One common type is a fluorescent black light, which looks like a regular fluorescent light tube but has a special coating inside that lets it give off UV light instead of normal visible light. When you turn on a black light, it may look purple, but this purple glow is not the UV light itself—it’s just a bit of normal light that escapes from the lamp.

There are also black lights made from regular light bulbs that have a special filter to block out most normal light, letting only UV light pass through. These are often used for things like checking if pets have wet spots, or for special lighting effects.

Main article: Mercury-vapor lamp

Mercury-vapor lamps are another way to create UV light. These lamps contain mercury and a gas called argon, and when they’re turned on, they give off strong UV light that can help kill germs on surfaces. This makes them useful in places like laboratories and food-processing factories where keeping things clean is very important.

A 380 nanometer UV LED makes some common household items fluoresce.

Other types of lamps, such as gas-discharge lamps and special LED lights, can also create UV light for scientific tools and other purposes. LEDs that give off UV light are becoming more common and are used for things like curing glue quickly, checking if money is fake, and even in some medical tools.

Main article: Gas-discharge lamp

Special lamps called gas-discharge lamps can create UV light at very specific wavelengths, which is helpful for scientific research. There are also UV LEDs, which are small lights that can give off UV light without being very hot. These are used for things like checking if money is fake and curing certain types of glue.

Main article: Excimer laser

Lasers can also be made to give off UV light. These UV lasers are used in many areas, such as making tiny parts for computers, engraving, and even in some medical treatments.

The very short-wavelength UV light, called vacuum ultraviolet (VUV), can be made using special methods with lasers and gases. This type of UV light is used in very advanced research and technology, like making tiny patterns for computer chips.

Lasers can also create a type of UV light called extreme ultraviolet (EUV) by heating certain gases to very high temperatures. This EUV light is used in making the smallest parts for computer chips, and it can be made without needing very large and expensive machines called synchrotrons.

Human health-related effects

Further information: Health effects of sunlight exposure

Ultraviolet (UV) radiation has both good and bad effects on our health. A little bit of sun helps our bodies make vitamin D, which we need to stay healthy. But too much sun can hurt us.

UV light can help treat some skin problems, like psoriasis and eczema. It can also help with other conditions like jaundice and vitiligo.

Too much UV light can hurt our eyes and skin. It can cause sunburn and may even lead to skin cancer over time. UV light can also make some skin diseases worse. Wearing sunglasses and using sunscreen can help protect us from the harmful parts of UV radiation.

Degradation of polymers, pigments and dyes

Main article: UV degradation

UV damaged polypropylene rope (left) and new rope (right)

Ultraviolet light can damage plastics and materials when they are exposed to sunlight for a long time. This can make things look faded, crack, or become weaker. Adding special UV blockers can help prevent this damage.

Some materials, like certain strong fibers and paints, are especially sensitive to UV light. They can lose their color or strength. That’s why museums often protect valuable paintings and old fabrics by covering them with special glass or curtains to keep the harmful UV light away.

Applications

Ultraviolet radiation has many useful applications because it can create chemical reactions and make certain materials glow. Here are some key uses:

Very short UV wavelengths (13.5 nm) are used in advanced technology for making tiny parts in electronic devices.
Wavelengths from 230 to 400 nm help track items like labels and are used in tools that can see things that are hidden.
Wavelengths around 260 nm can clean surfaces and water by harming germs.
Wavelengths from 280 to 400 nm are used to see detailed images of cells and other tiny structures in medicine.
Wavelengths around 365 nm help cure glues and protect materials from damage.

Photography

Ultraviolet light can be used in special types of photography. Regular camera lenses often block ultraviolet light, but special cameras and filters can capture these images. This type of photography is useful for checking for injuries, reading old documents, and studying stars and space through telescopes.

Electrical and electronics industry

Ultraviolet light helps detect problems in electrical equipment and can erase memory in certain computer chips.

A portrait taken using only UV light between the wavelengths of 335 and 365 nanometers

Fluorescent dye uses

Some dyes glow under ultraviolet light and are used to make paper and fabrics appear brighter. They are also used to protect important documents like money and passports from being copied. These glowing dyes can also help find problems in materials or identify attackers using special sprays.

Analytic uses

Forensics

Ultraviolet light helps find important clues at crime scenes, like tiny drops of body fluids. It can also help read old or damaged writing and verify the authenticity of valuable items.

Enhancing contrast of ink

Special imaging techniques using ultraviolet light can reveal hidden writing on old documents, like burned papyrus or damaged books.

Aurora at Jupiter's north pole as seen in ultraviolet light by the Hubble Space Telescope

Sanitary compliance

Ultraviolet light is used to check if surfaces are clean in places like hotels and factories.

Chemistry

Ultraviolet light helps scientists study chemicals and is used to detect pollution in water and air.

Material science uses

Fire detection

Ultraviolet sensors can detect fires by the special light they give off, which helps in making safer fire alarms.

A bird appears on many Visa credit cards when they are held under a UV light source.

Photolithography

Ultraviolet light is used to create tiny patterns on materials, which is important for making computer chips and other electronic parts.

Polymers

Ultraviolet light can harden certain glues and coatings quickly, which is useful for fixing things like glass or plastic. It can also change the surface of materials to make them stick better.

Biology-related uses

Air purification

Ultraviolet light is used in air conditioners to clean the air by harming tiny living things like bacteria and viruses.

Sterilization and disinfection

Ultraviolet lamps are used to clean and sterilize tools in labs and medical places. They are also used to clean water and food products.

Biological

Some animals, like birds and insects, can see ultraviolet light, which helps them find food or mates. Ultraviolet light is also used in scientific research to study living things.

Therapy

Ultraviolet light is used to treat skin conditions like psoriasis.

Herpetology

Reptiles need ultraviolet light to stay healthy, especially to make important vitamins. Special lights are used in enclosures to give them the right amount of light.

Evolutionary significance

Scientists believe that ultraviolet radiation helped shape early life on Earth. When simple organisms, called prokaryotes, lived near the ocean surface before the ozone layer protected the planet, UV radiation could damage their DNA. This damage often killed the cells. However, a few survived by developing special enzymes that could fix the damaged DNA. These enzymes are thought to be the ancestors of important proteins we have today that help cells divide correctly.

Some scientists also think that high levels of UV radiation may have played a role in big changes in Earth's history, when many living things died out.

Photobiology

Main article: Photobiology

Photobiology is the study of how different types of energy from light affect living things. It looks at how radiation, including ultraviolet (UV) light, can help or harm organisms. UV light has energies ranging from about 3 to 30 electronvolts, which is part of what photobiology studies.